1. Field of the Invention
The present invention relates to a laser mask and method of crystallization using the same, and particularly, to a laser mask and method of crystallization using the same which crystallizes active regions of thin film transistors without a shot mark.
2. Discussion of the Related Art
Recently, as information displays, especially portable information displays, have drawn great attention, thin and lightweight flat panel display (FPD) devices which can replace existing Cathode Ray Tubes (CRTs) have been actively researched and commercialized. Particularly, of these FPD devices, liquid crystal display (LCD) devices, as a device for displaying images by using optical anisotropic properties of liquid crystal, are widely used for notebook computers or desktop monitors due to their superior resolution, color rendering capability, image quality or the like.
An active matrix (AM) driving method is generally used for driving the LCD devices which use an amorphous silicon thin film transistor (TFT) as a switching element in a pixel unit.
General ideas of the amorphous silicon thin film transistor technique were established by LeComber, et al. in Britain in 1979 and were commercialized as a three-inch liquid crystal portable television in 1986. Recently, a large dimension TFT-LCD device with its size being more than 50 inches has been developed.
However, due to its lower electrical mobility (<1 cm2/Vsec), the amorphous silicon thin film transistor has a limit on its use for peripheral circuits which require a high speed operation at greater than 1 MHz. Thus, researches for simultaneously integrating a pixel unit and a driving circuit unit on a glass substrate using a polycrystalline silicon thin film transistor which has a field effect mobility greater than that of the amorphous silicon thin film transistor have been actively carried out.
The polycrystalline silicon thin film transistor technique has been applied to a small module such as camcorders, since a liquid crystal display color television was developed in 1982. According to the polycrystalline silicon thin film transistor technique which supports low photosensitivity and high electric field effect mobility, driving circuits can be fabricated directly on a substrate.
Increased mobility can improve an operation frequency of the driving circuit unit which determines the number of pixels that can be driven, and fine minuteness of a display device can be thereby facilitated. In addition, a distortion of a transfer signal is reduced by a decrease of a charging time for a signal voltage of the pixel unit, thereby expecting an improvement of an image quality.
Furthermore, the polycrystalline silicon thin film transistor can be driven under 10V, compared with the amorphous silicon thin film transistor requiring a high driving voltage (i.e., approximately 25V), so as to advantageously reduce power consumption.
On the other side, the polycrystalline silicon thin film transistor can be fabricated by depositing a polycrystalline silicon thin film directly on a substrate or by depositing a amorphous silicon thin film on a substrate that is then crystallized by a thermal treatment. In particular, in order to use a low-cost glass substrate, low temperature processes are required, and in order to use the polycrystalline silicon thin film transistor for a device in the driving circuit unit, a method for improving a field effect mobility of the thin film transistor is required.
In general, the thermal treatment methods for crystallizing an amorphous silicon thin film include a solid phase crystallization (SPC) method and an excimer laser annealing (ELA) method.
The SPC method, for instance, forms a polycrystalline silicon thin film at a temperature of approximately 600° C. In this SPC method, after forming an amorphous silicon thin film on a glass substrate, the amorphous silicon thin film is crystallized by performing a thermal treatment for several up to tens of hours at approximately 600° C. A polycrystalline silicon thin film obtained by the SPC method generally has comparatively large-size grains of about several μm. However, the grains contain many defects therein. These defects are known to have negative influences on the capability of the thin film transistor, although not as bad as grain boundaries in the thin film transistor.
The ELA method is a typical method for fabricating a polycrystalline silicon thin film at a low temperature. In this ELA method, an amorphous silicon thin film is crystallized by instantaneously irradiating a high energy laser beam onto the amorphous silicon thin film for a time of tens of nsec (nanoseconds). In this method, the amorphous silicon thin film is melted and crystallized in a very short time, so that the glass substrate is not damaged.
Moreover, a polycrystalline silicon thin film fabricated by using the excimer laser has excellent electrical characteristics, compared to a polycrystalline silicon thin film fabricated by a general thermal treatment method. For instance, a field effect mobility of an amorphous silicon thin film transistor is about 0.1˜0.2 cm2/Vsec, and that of an polycrystalline silicon thin film transistor fabricated by a general thermal treatment method is about 10˜20 cm2/Vsec. A field effect mobility of a polycrystalline silicon thin film transistor fabricated by using the excimer laser method is more than 100 cm2/Vsec.
A crystallization method using a laser will now be explained in detail. FIG. 1 is a graph showing a size of the grain of a crystallized silicon thin film with respect to a laser energy density used to form the crystallized silicon thin film.
Referring to FIG. 1, as the laser energy density increases, the grain size of the polycrystalline silicon thin film increases in the first region I and the second region II. However, in the third region III, when an energy density above a specific energy density Ec is irradiated, the grain size of the polycrystalline silicon thin film drastically decreases. That is, a crystallization mechanism for the silicon thin film becomes different according to the irradiated laser energy density, which will now be explained in detail.
FIGS. 2A to 2C, 3A to 3C and 4A to 4C are cross-sectional views illustrating silicon crystallization mechanisms according to laser energy densities in the graph of FIG. 1. The drawings illustrate sequential crystallization processes according to each laser energy density. A crystallization mechanism of amorphous silicon by a laser annealing is affected by various factors, such as laser irradiation conditions (i.e., laser energy density, irradiation pressure, substrate temperature or the like), physical and geometrical characteristics (i.e., absorption coefficient, thermal conductivity, mass, impurity containing degree, thickness or the like) of the amorphous silicon thin film and so on.
First, as illustrated in FIGS. 2A to 2C, the first region I of the graph shown in FIG. 1 is a partial melting region, and an amorphous silicon thin film 12 is crystallized only down to the dotted line. A size of a grain 30 formed at this time is about hundreds Å.
That is, when a laser beam of the first region I is irradiated onto an amorphous silicon thin film 12 on a substrate 10 on which a buffer layer 11 is formed, the amorphous silicon thin film 12 melts. At this time, strong laser energy is irradiated at a surface of the amorphous silicon thin film 12 which is directly exposed to the laser beam, and relatively weak laser energy is irradiated at a lower portion of the amorphous silicon thin film 12. As a result, the amorphous silicon thin film 12 melts only down to a certain portion so as to achieve a partial crystallization.
In the laser crystallization method, processes of a crystalline growth include primary melting in which the amorphous silicon surface layer is melted by a laser irradiation, second melting in which a lower layer is melted by a latent heat generated during the solidification of the primarily melting layer, and a crystal growth by the solidification. These crystal growth processes will now be described in detail.
An amorphous silicon thin film on which a laser beam is irradiated has a melting temperature of more than 1000° C. and primarily melts into a liquid state. Afterwards, because there occurs a great temperature difference between the primarily melting layer and the lower silicon and substrate, the primarily melting layer cools fast until solid phase nucleation and solidification are occurred. The melted layer by the laser bream irradiation remains until the solid phase nucleation and the solidification are completed. Thus, the melting state lasts for a long time when the laser energy density is high or thermal emission to the outside is low at a range where ablation does not occur. Furthermore, the primarily melting layer melts at a temperature (1000° C.) lower than the melting temperature (1400° C.) for crystalline silicon, and thus the melted layer cools and maintains a super-cooled state where the temperature is lower than the phase transition temperature. When the super-cooled state is great, namely, when the melting temperature of the thin film is low or the cooling speed is fast, a nucleation rate at the time of the solidification becomes great so as to achieve fine crystal growth.
Once the primarily melting layer cools as the solidification starts, the crystals grow in an upward direction from a crystal nucleus. As the primarily melting layer transforms its phase from a liquid state to a solid state, the latent heat is discharged. As a result, the secondary melting begins to melt the lower amorphous silicon thin film in the solid state and then the solidification occurs again. Thus, this procedure is repeated to grow crystals. The lower secondarily melting layer is more supper-cooled than the primarily melting layer, and accordingly the nucleation rate increases to make the size of the crystal smaller.
Therefore, an effective method of improving characteristics of the crystallization is to reduce the cooling speed. The cooling speed can be reduced by preventing the heat of absorbed laser energy from being emitted to the exterior, examples of which are heating the substrate, irradiating double beam, inserting a buffer insulating layer or the like.
FIGS. 3A through 3C are cross-sectional views sequentially showing the silicon crystallization mechanism corresponding to the second region II of the graph shown in FIG. 1. The second region II indicates a near-completely melting region.
As can be seen in the drawings, a polycrystalline silicon thin film has relatively large grains (30A to 30C) of about 3000 to 4000 Å and is formed down to an interface of a lower buffer layer 11. That is, when a nearly complete melting energy, but not a complete melting energy, is irradiated on the amorphous silicon thin film 12, the amorphous silicon thin film 12 adjacent to the buffer layer 11 melts. At this time, solid seeds 35 that have not been melted exist at the interface between the melted silicon thin film 12′ and the buffer layer 11. The seeds act as a crystallization nucleus to induce a lateral growth, thereby forming relatively large-size grains 30A to 30C.
However, because this crystallization method is possibly used only when the laser energy is such that the non-melted solid seeds 35 can remain on the interface with the buffer layer 11, the process window (process margin) is disadvantageously very limited. In addition, the solid seeds 35 are generated non-uniformly and accordingly the crystallized grains 30A to 30C of the polycrystalline silicon thin film may have different crystallization directions and different crystallization characteristics.
FIGS. 4A through 4C are cross-sectional views illustrating the crystallization mechanism of a completely melting region corresponding to the third region III of the graph shown in FIG. 1.
As can be seen in the drawings, very small grains 30 are irregularly formed with an energy density corresponding to the third region III. That is, when the laser energy density is more than a specific energy density level Ec, sufficient energy is applied to the amorphous silicon thin film 12 to completely melt the amorphous silicon thin film 12. As a result, solid seeds which may be grown to grains do not remain thereon. Afterwards, the silicon thin film 12′ which has been melted by receiving the strong laser energy undergoes a rapid cooling process, which leads to a nucleus 30 generation and the fine grains 30.
On the other hand, an excimer laser annealing method employing a pulse type laser is generally used for the laser crystallization. However, a sequential lateral solidification (SLS) method in which the crystallization characteristics are dramatically improved by growing grains in a horizontal direction has been proposed.
The sequential lateral solidification (SLS) method utilizes the fact that the grain grows from an interface between liquid phase silicon and solid phase silicon in a perpendicular direction of the boundary surface. Here, the SLS is such a crystallization method in which the size of the silicon grain can be increased by appropriately controlling the size of the laser energy and an irradiation range of a laser beam and thus growing the grains laterally as long as a predetermined length.
As the SLS is an example of lateral solidification, the crystallization mechanism for the lateral solidification will be now described with reference to FIGS. 5A through 5C. FIGS. 5A through 5C are cross-sectional views sequentially showing a crystallization process according to the lateral solidification.
First, as illustrated in FIG. 5A, when laser energy above an energy density at which an amorphous silicon thin film 112 completely melts (i.e., the third region III of the graph shown in FIG. 1) is irradiated onto an amorphous silicon thin film 112, the portion of the amorphous silicon thin film 112 onto which the laser energy has been irradiated completely melts. In this method, a laser-irradiated region and a laser non-irradiated region can be formed by using a patterned mask.
At this time, as illustrated in FIGS. 5B and 5C, because sufficient energy is irradiated onto the amorphous silicon thin film 112, the amorphous silicon thin film 112 completely melts. However, because the laser beam is irradiated with certain intervals, the silicon thin film 112 at the laser non-irradiated region and the solid silicon existing at the interface with the melted silicon thin film 112′ work as nucleuses for crystal growth.
That is, the melted silicon thin film 112′ cools from the left/right surfaces, namely, from the laser non-irradiated region, immediately after the laser beam is completely irradiated. This is because the solid phase amorphous silicon thin film 112 positioned at left/right surfaces has a higher heat conductivity than the buffer layer 111 or the glass substrate 110 at the lower portion of the silicon thin films 112 and 112′.
Therefore, the melted silicon thin film 112′ first reaches a nucleation temperature at the interface between the solid phase and the liquid phase positioned at the left/right sides, rather than the central portion, thereby forming a crystal nucleus at the corresponding portion. After the crystal nucleus is formed, the grains 130A and 130B horizontally grow from a low-temperature side to a high-temperature side, namely, from the interface to the central portion. Thus, large-size grains 130A and 130B are formed by the lateral crystal growth, and the process window (process margin) is advantageously wide (not limited) because the process is performed with an energy density of the third region III.
However, the SLS method performs the crystallization by infinitesimally and repeatedly moving the mask or a stage in order to increase the size of the grains. Thus, it may take a long time to crystallize a large-size silicon film, and accordingly the whole process time may be lengthened and the process yield may become lower.